The present invention relates to a process for operating a group of regenerative hot blast stoves for heating cold blast from a constant supply source and for supplying the thus formed hot blast at a selectively variable predetermined temperature and at a constant volume to a blast furnace. The present invention is particularly directed to such a process wherein the temperature of the hot blast is regulated to the temperature required at the blast furnace in a two-stage operation. In a first stage, hot blast issues from more than one of the stoves, but at relatively cooler and warmer temperatures. These different temperature blasts are mixed, and this mixing is regulated as a function of a first reference temperature which is greater than the temperature required at the blast furnace. In a second stage, this hot blast mixture is admixed with cold blast as a function of a second reference temperature which equals the temperature required at the blast furnace.
As is well known, hot blast, i.e. air which is heated to a high temperature in hot blast stoves, is supplied to a blast furnace for the purpose of carrying out combustion and reduction operations within the blast furnace. Typically a plurality of hot blast stoves are alternately regenerated and employed for the purpose of supplying such hot blast. Conventionally such hot blast stoves are operated in accordance with three different processes.
When the installation is operated in accordance with the "parallel" process, half of the hot blast stoves are operated simultaneously for a predetermined length of time to supply hot blast to the blast furnace, while the other half of the hot blast stoves are operated during the same predetermined length of time to be heated. When the first stoves are switched from blasting to heating, then the other half of the stoves are simultaneously switched from heating to blasting. During the operation of the hot blast stoves in accordance with the parallel process, the temperature of the hot blast which is supplied to the blast furnace is regulated by admixing therewith cold blast as a function of the temperature required at the blast furnace.
The second known process is the "tandem" process, and when the installation is operated under this process, each hot blast stove is operated under blasting for a predetermined length of time and is then operated under heating for a further predetermined length of time. The cycling of the blast periods of the hot blast stoves is in a tandem manner with normally only one hot blast stove operating under blasting at a given time, or when a plurality of hot blast stoves are operating under blasting, their blasting cycles correspond in time with respect to each other. During the operation of the hot blast stoves in accordance with the tandem process, the hot blast supplied to the blast furnace is also admixed with cold blast as a function of the temperature required at the blast furnace.
Both of the parallel and tandem processes are recognized to have the disadvantage in that each of the hot blast stoves is required to operate throughout the entire blasting cycle thereof at a temperature higher than the temperature required at the blast furnace.
This disadvantge can however be overcome by operating the hot blast stoves in accordance with the third known type of process, i.e. the "staggered-parallel" process. When operating the installation in accordance with the staggered-parallel process, each of the hot blast stoves is operated under blasting for a predetermined length of time and then operated under heating for a predetermined length of time. However, the cycles of the hot blast stoves are staggered in time with respect to each other such that at any given time hot blast issues from more than one of the stoves but at different temperatures. The temperature of the hot blast issuing from the stoves is regulated to the temperature required at the blast furnace in a two-stage operation. In a first stage, the relatively cooler and warmer hot blasts issuing from those stoves operating under blasting are mixed, and this mixing is controlled as a function of a first reference temperature which is greater than the temperature required at the blast furnace. During a second stage, the hot blast mixture has admixed thereto cold blast, and this admixing operation is controlled as a function of a second reference temperature corresponding to the temperature required at the blast furnace.
However, operation of the installation in accordance with the staggered-parallel process suffers from a further disadvantage which has not been solved in the art prior to the present invention.
Specifically, when it is necessary to drastically reduce the second reference temperature over a short period of time, the second stage temperature regulation operation inherently requires a greater amount of cold blast. This results in the second stage temperature regulation operation using at least a portion of the cold blast which would otherwise be supplied to those hot blast stoves which are operating under blasting. Therefore, the hot blast which issues from the hot blast stoves is inherently at an increased temperature which is at a sufficiently high level that it is no longer possible to perform the first stage temperature regulation operation. That is, the relatively warmer and cooler hot blasts issuing from the hot blast stoves are all at a temperature such that it is impossible to regulate the mixture of such blasts to the first reference temperature.
The result of this phenomenon is that the first stage temperature regulation operation becomes completely inoperative, and the entire temperature regulation must be achieved only by the second stage temperature regulation operation. Thus, when it becomes necessary due to the requirements of the blast furnace to drastically lower the second reference temperature, the hot blast stove installation cannot be operated under the staggered-parallel process, and thus the advantages thereof are lost.